Using a tape measure is simple and intuitive once you get the hang of it.

Recruit a helper
Unless you’re measuring a distance that’s shorter than your arm span, taking a measurement is much easier if you have someone to help you.

Define what you’re measuring
Figure out what you’re actually measuring, as in what two points you’re using as the beginning and the terminus of your measurement. Mark them using an easily wiped-off instrument like chalk or a grease pen.